Liquid or gaseous nitrogen is conventionally prepared by cryogenic distillation of air, which is carried out in a cold box containing one or more distillation columns.
The nitrogen thus obtained exhibits a purity which is sufficient for most of its applications.
However, some activity sectors such as the electronics industry, especially for the manufacture of semiconductors or other components, the pharmaceutical industry, analysis or research, require nitrogen of higher purity.
Depending on the application in question, the nitrogen must, in particular, be substantially free from at least one of the impurities hydrogen, oxygen and carbon monoxide.
Maximum contents of these impurities of the order of a few ppb (parts per billion by volume) are then required.
The impurities hydrogen and carbon monoxide are conventionally removed from the air to be distilled or from the gaseous nitrogen resulting from the cryogenic distillation. To do this, purification processes have already been proposed, according to which said impurities can be reacted with a sufficient quantity of oxygen in the presence of a catalyst, to form water and carbon dioxide which are subsequently removed, for example by adsorption. Processes of this type are described, for example, in U.S. Pat. No. 4,869,883 and in Patent Applications EP-A-454,531 and FR-A-2,690,357.
Patent application JP-A-05/079754 describes a process for removing impure carbon dioxide, water and carbon monoxide from liquid nitrogen. According to this process, liquid nitrogen from a distillation column is introduced into an absorbing column, the liquid nitrogen then passes through said column and returns to the distillation column. The adsorbent used is a non-exchanged A type zeolite.
Another process for removing carbon monoxide from liquid nitrogen by adsorption with non exchanged zeolites is described in U.S. Pat. No. 5,333,463.
It has also been proposed to remove oxygen or carbon monoxide from cold gaseous nitrogen by adsorption. Such a process is described, for example, in U.S. Pat. No. 4,746,332.
It appears that the processes for the preparation of high-purity nitrogen are all--as far as the applicant is aware--performed on a gas, usually air or the nitrogen to be purified. Because of the large volumes of gas to be treated, the devices needed for making use of these processes are generally large in size, in particular when said gas consists of air.
Furthermore, with a view to reducing costs and investments, it is commonplace to store and to transport nitrogen in the liquid state. If it is desired to prepare high-purity liquid nitrogen, it is then necessary, in a first step, to purify the air to be distilled or gaseous nitrogen and then, in a second step, to liquefy the purified gaseous nitrogen.
However, the change from the gaseous state to the liquid state involves stages during which the nitrogen may be accidentally recontaminated by impurities, chiefly oxygen, and sometimes carbon monoxide and/or hydrogen. Furthermore, even when the liquefaction is carried out with great care, liquid nitrogen can still be recontaminated with said impurities when it is transferred from its storage or transportation site to another site of this type.
Moreover, a purification performed on a gas produces a large pressure drop. It would therefore be appropriate to perform this purification directly on liquid nitrogen to avoid this pressure drop.